RedXIII added nothing? I think the entire visit to Cosmo Canyon disagrees with you. And while not every character in a game has to add a lot, for people to like them they need to add something, even if that's only comic relief or general awesomeness.

And for me, it's not that Penelo specifically didn't add much; in my opinion, a lot of FFXII was underdeveloped, so I had trouble caring about other characters and parts of the plot as well.